Use config API to access allow_browser_cache

[dregad]

Until now $g_allow_browser_cache config was handled in a non-standard way in in http_caching_headers(), i.e. by referencing it as a global variable instead of retrieving its value with config_get_global().

Moreover, the setting was commented out in config_default_inc.php (i.e. not defined) instead of being set to OFF.

This follows up on discussion in PR #1925.

Fixes #33482

[dregad]

Please read https://mantisbt.org/bugs/view.php?id=33482#c68427 as part of review

[atrol]

I am fine with the changes.

As a side note: Using config_get_global is about 7 times (xdebug.mode = debug) / 11 times (without xdebug) slower than direct access of the variable.

[dregad]

Thanks for the feedback @atrol

Using config_get_global is about 7 times (xdebug.mode = debug) / 11 times (without xdebug) slower than direct access of the variable.

Which is to be expected of course, although I'm surprised it's actually faster with Xdebug than without...

I just followed @vboctor's note in #1925 (comment)

Update the code fetching them to use the standard config_get_global() method.

[atrol]

I'm surprised it's actually faster with Xdebug than without...

@dregad It's not faster with Xdebug.
Seems my formulation was not clear.

As a side note: Using config_get_global is about 7 times (xdebug.mode = debug) / 11 times (without xdebug) slower than direct access of the variable.

I compared two times config_get_global calls with direct access of the variable.
With Xdebug enabled, config_get_global is about 7 times slower than direct access of the variable, (e.g. 7 seconds vs. 1 second)
With Xdebug disabled, config_get_global is about 11 times slower than direct access of the variable. (e.g. 5.5 seconds vs. 0.5 seconds)

[dregad]

Thanks for the clarification.